The invention relates to means for producing cooling by thermal radiative transfer, particularly to a material utilized in a wavelength-selective radiative cooling system, and more particularly to such a material comprising an infrared-reflective substrate coated with magnesium oxide in a ceramic form and/or lithium fluoride in a polycrystalline form.
A well known principle of physics is that a good absorber of electromagnetic radiation is also a good radiator, and the net amount of power that is radiated depends on the difference between the temperature of the radiating body and the temperature of its surroundings. It is also recognized that there are no atomic or molecular resonances in the air to absorb radiation in the wavelength range between 8-13 microns. Therefore, a selectively emitting body radiating only in that range, exposed to the clear sky, looks through the "window" in the atmosphere and "sees" a very low temperature. It follows that low temperatures can be reached, if other heat gains are small. In contrast, ordinary spectrally non-selective emitters receive "warm" radiation from the atmosphere in the spectral ranges 5 to 8 and 13 to 40 microns. The attainment of low temperatures is precluded by the absorption of this radiation. In a clear dry climate, either a selective or non-selective radiator can produce about 135 Wm.sup.-2 of cooling at air temperature. In this case, an ordinary non-selective radiator can (theoretically) cool to about 22.degree. C. below air temperature; an ideal selective radiator can cool to 53.degree. C. below air temperature (based on measured 8 to 13 micron sky radiances).
The basic idea of a thermally selective radiator is to permit transfer of thermal infrared radiation (5 to 40 microns) only in that part of the spectrum which corresponds to the 8-13 micron transparency "window" of the atmosphere (through which cold space can be seen), while preventing transfer in the rest of the spectrum. For radiator temperatures below ambient, such selective radiators which are exposed to the sky can produce more cooling than an infrared-black radiator which cannot reject the "warm" radiation in the spectral bands 5-8 and 13-40 microns.
A thermally selective radiator is generally composed of an infrared-reflective substrate, such as a metal film, coated with a material which has a large infrared absorption in the band (8-13 microns) and little absorption elsewhere. Materials which have been previously suggested for the spectrally selective coatings are Tedlar, a plastic made by DuPont; TPX, a plastic made by Mitsui Petrochemical Industries LTD; silicon monoxide silicon oxynitride; aluminum oxide; and magnesium oxide.
Various approaches have been directed to selective radiation cooling and materials therefor. These prior efforts are exemplified by U.S. Pat. Nos. 3,043,112 issued July 10, 1962 to A. K. Head; 3,310,102 issued Mar. 21, 1967 to F. Trombe; 3,671,286 issued June 20, 1972 to R. F. Fischell; 4,147,040 issued April 3, 1979 to G. Altman; and 4,323,619 issued April 6, 1982 to V. Silvestrini et al. Also, the following articles illustrate prior efforts in this field: "Perspectives Sur L'utilisation Des Rayonnements Solaries Et Terrestres Dans Centain Regions du Monde", F. Trombe, Rev. Gen. Thermique 6, 1285-1314, 1967; The Radiative Cooling of Selective Surfaces, S. Catalanotti et al, Solar Energy, Vol. 17, pp 83-89, 1975; Selective Radiation Cooling "Another Look", D. Michell, CSIRO Division of Tribophysics, University of Melbourne, Parkville, Victoria, Australia, September 1976; Radiative Cooling To Low Temperatures: General Considerations And Application To Selectively Emitting SiO Films, C. G. Granqvist et al, J. Appl. Phys. 52, 4205-4220, 1981; and "Infrared Optical Properties Of Electron-Beam Evaporated Silicon Oxynitride Films", Applied Optics 22, 3204-3206, 1983.
One of the problems with the materials primarily utilized in the prior known selective radiation cooling efforts is that these materials absorb some radiation at wavelengths outside the 8 to 13 micron band.
The above-referenced patent to Trombe disclosed magnesium oxide (along with other white pigments: CaO, CO.sub.3 Ca, TiO.sub.2, ZnO.sub.2) as a radiator material, without reference to its potential 8 to 13 micron selectivity. (In Trombe's patent the word "selective" is often used to denote materials which are white in the solar spectrum, 0.3 to 3 microns, and black in the thermal spectrum, 5 to 40 microns; whereas, the word "selective" is used herein to denote materials which are black between 8 and 13 microns and white outside this range.) However, the subsequent above-referenced paper by Trombe recognizes the potential for 8-13 micron selective cooling by the use of MgO in the form of single crystals. A need has existed in the art for materials not in the form of single crystals which can be effectively utilized in selective radiation cooling systems, and particularly such materials that are reflective in the 13-40 micron wavelength range.
Therefore, it is an object of this invention to provide a material for selective radiative cooling systems.
A further object of the invention is to provide a selective radiative cooling material which is reflective at wavelengths longer than 13 microns.
A still further object of the invention is to provide a selective radiation cooling material which is non-absorptive at wavelengths shorter than 8 microns (radiation scattering is permissible), absorptive from 8 to 13 microns, and reflective at wavelengths longer than 13 microns.
Another object of the invention is to provide a material for selective radiative cooling, which is reflective at wavelengths over 13 microns and consisting of a reflective substrate or coating which is coated with magnesium oxide and/or lithium fluoride.
Another object of the invention is to provide a selective radiative cooling material consisting of a metallic substrate coated with MgO in a ceramic form and or LiF in a polycrystalline form.
Other objects of the invention will become readily apparent from the following description and the accompanying drawings.